A capital loss is the shortfall you make when you dispose of an asset for less than your total cost of acquiring and holding it. It applies to investments like shares, cryptoassets, funds and property that are held as capital rather than as trading stock.
The loss becomes real, or realised, when there is a disposal. Before that it is just an on-screen drop in value. A capital loss is the opposite of a capital gain.
What events create a capital loss?
In most systems a disposal is the trigger. That usually means you sell the asset for cash, but it can also include swapping one asset for another, gifting it, redeeming a fund or having it expire or be cancelled. In crypto, exchanging one token for a different token is often treated as a disposal for tax, even if you never touch cash. Rules vary by country and can change, so always check the current guidance where you live.
A loss can also arise if an asset becomes effectively worthless and you claim a disposal under your local rules. Some jurisdictions set conditions for treating an asset as worthless, or require evidence such as a delisting or liquidation notice.
Until a disposal happens any loss is unrealised. It affects your portfolio value and your risk, but it normally does not appear on a tax return.
Realised versus unrealised losses
Unrealised loss is the fall from your cost to the current market price while you are still holding the asset. It can reverse if the price recovers. It matters for risk management and margin, since lenders and brokers look at the current value of your collateral. It may also guide your decisions on whether to hold, add or exit.
Realised loss appears only when you dispose. It is the figure that can be used in tax calculations and year-end reporting. Many investors talk about loss harvesting, which is the practice of realising a loss to offset gains elsewhere in the portfolio. That can be rational, but it carries timing and execution risks. Selling to bank a loss means you are out of the market for at least some period, and repurchasing too quickly may clash with anti-avoidance rules in your jurisdiction.
Working out the loss: cost basis and adjustments
The basic formula is simple: net sale proceeds minus cost basis. If the result is negative, you have a capital loss. The detail sits in what counts as cost and what counts as proceeds.
- Proceeds are the amount you receive on disposal, after deducting directly related selling costs such as brokerage commission, exchange fees or stamp duties charged on the sale.
- Cost basis usually includes the purchase price plus directly related acquisition costs such as commissions, fees and taxes paid on purchase. For property, capital improvements may be added to the cost base in some regimes.
- Corporate actions can alter cost basis. Stock splits, consolidations, return of capital and bonus issues typically require you to adjust the per share cost figure. With funds, some countries require an average cost method. With shares, you may be allowed first in, first out or specific lot identification. The permitted method varies by country and sometimes by asset type.
Small example with numbers. You buy 100 shares at £20, paying £10 commission, so your total cost is £2,010. Later you sell the 100 shares at £17 and pay £10 to sell. Your net proceeds are £1,690. The capital loss is £1,690 minus £2,010, which equals £320.
Partial disposals need a method. Say you bought 100 shares at £20 and another 100 at £14. You later sell 100 at £16. If you must use first in, first out, the sale is matched to the £20 lot, giving a £4 per share loss, adjusted for costs. If specific identification is allowed and you choose the £14 lot, you would have a £2 per share gain instead. This is why local cost basis rules matter.
For crypto, the same logic applies. If you swap 1 coin of Token A for 2 coins of Token B, you treat the value received for Token A as your proceeds at the time of the swap. Your cost basis is what you originally paid for Token A, including any fees. The difference is your gain or loss in your base currency.
Using losses against gains and the traps to avoid
Many jurisdictions allow realised capital losses to offset realised capital gains of the same tax year. If losses exceed gains, some systems let you carry the excess forward to reduce future gains, sometimes indefinitely, sometimes for a limited number of years. A few allow a portion of net capital losses to offset certain types of income each year. The details, order of netting and any caps differ by country and can change.
Holding period can matter. In places that distinguish short term and long term gains, losses may first be netted within each bucket before being combined. Again, the precise sequence and rates are set locally.
Anti-avoidance rules are common. Many tax codes have provisions that ignore a capital loss if you dispose of an asset and buy back the same, or a substantially similar, asset within a set period. In share markets this is often called a wash sale rule. In the UK there are rules that match sales to repurchases within specific windows. Timeframes, what counts as similar and how the disallowed loss is treated differ by jurisdiction. The broad idea is to prevent claiming a paper loss while maintaining the same economic exposure.
Admin matters. To claim a loss you usually need records that show dates, quantities, prices and fees, plus any adjustments from corporate actions or token swaps. If you trade across several brokers or exchanges, collating this data early saves effort later.
None of this is personal advice. Tax treatment depends on your circumstances, the asset and your country’s law, which can change.
Where you will see capital losses discussed
Broker statements and portfolio dashboards often show realised and unrealised profit and loss lines. At year end, your account documents may list realised gains and losses by instrument, sometimes with a cost basis method noted. Fund reports sometimes mention realised losses they carry forward, which can affect future distributions.
Company financial statements use different language. An operating loss is not a capital loss. Impairments and write downs are accounting charges that reduce the carrying value of assets in the accounts. They are related ideas but sit in company reporting, not in an individual investor’s capital gains schedule.
In market conversations, traders might say they have locked in a loss or that a position is underwater. Only when they close or reduce the position does that become a realised capital loss for tax purposes.
How capital loss fits into portfolio decisions
Capital losses are part of the investing landscape. Prices move around and some positions will not work out. The practical question is how you respond. Some investors use stop losses to limit downside. Others may average down, which reduces the average entry price but increases the capital at risk. Neither approach guarantees better outcomes.
Loss harvesting can tidy up a tax year, but the market risk of being out, the bid ask spread and trading costs all eat into the benefit. If you do repurchase, be mindful of anti-avoidance rules and whether you are simply re-entering the same exposure without a clear thesis.
For context, the flip side of a capital loss is a gain. If you are working out how losses and gains combine, see our page on capital gains tax for how many systems approach the calculation at a high level.